Conventionally, as a photo-printing device for printing an image recorded on a film onto a printing paper, there have been proposed various kinds of an analog printer for directly exposing the printing paper and various kinds of a digital printer which causes a CCD (charge coupled device) to read the image recorded on the film so as to expose the printing paper in accordance with the obtained image data. Particularly, the digital printer is advantageous in that: it is possible to perform color correction and density correction etc., that cannot be realized by the analog printer, by using the digital printer in combination with an image processing device for performing the image process, such as the color correction and the density correction, with respect to the image data, and it is possible to obtain the image which satisfies needs of a customer easily and quickly. Thus, such a digital printer is widely used now.
Such a digital printer is used in combination with an image processing device for performing the image process, such as the color correction and the density correction, with respect to the image data, so that it is possible to perform color correction and density correction etc., that cannot be realized by the analog printer. Thus, the digital printer is advantageous in that: by using the digital printer in combination with the foregoing image processing device, it is possible to obtain the image which satisfies needs of a customer easily and quickly.
As to the image processing device, interpolating pixels are calculated in accordance with a linear interpolating (bi-linear) method or a three dimensional interpolating (bi-cubic) method so as to vary the number of pixels in a case where the digital image is scaled up and down.
In both cases of the linear interpolating method and the three dimensional interpolating method, data of pixels in a scaled-up image is calculated by performing interpolation in accordance with pixel data of an image which has not been scaled up.
Here, the linear interpolating method is detailed. That is, description is given on a case where: a pixel value of an original image is indicated by P(i,j) (i,j is a coordinate value), and a pixel value Q(x,y) at a time when the original image is scaled up (down) by r times is calculated. Note that, when r>1, the scaling-up process is performed, and when 0<r<1, the scaling-down process is performed. At this time, Q(x,y) is calculated in accordance with the following expression (1).
                              Q          ⁡                      (                          x              ,              y                        )                          =                                            (                              1                -                t                            )                        ⁢                          {                                                                    (                                          1                      -                      s                                        )                                    ⁢                                      P                    ⁡                                          (                                              i                        ,                        j                                            )                                                                      +                                  s                  ⁢                                                                          ⁢                                      P                    ⁡                                          (                                                                        i                          +                          1                                                ,                        j                                            )                                                                                  }                                +                      t            ⁢                          {                                                                    (                                          1                      -                      s                                        )                                    ⁢                                      P                    ⁡                                          (                                              i                        ,                                                  j                          +                          1                                                                    )                                                                      +                                  s                  ⁢                                                                          ⁢                                      P                    ⁡                                          (                                                                        i                          +                          1                                                ,                                                  j                          +                          1                                                                    )                                                                                  }                                                          (        1        )            
Note that, in the foregoing expression (1), i=[x/r], j=[y/r]([a] indicates the maximum integer not more than a), s=x/r−i, t=y/r−j.
FIG. 8 illustrates a relationship in the expression (1). As shown in FIG. 8, Q(x,y) is a pixel value corresponding to a specific point in an internal area of a square surrounded by four points P(i,j), P(i+1,j), P(i,j+1), and P(i+1,j+1) of the original image. The specific point divides P(i,j) and P(i+1,j) so that s:1−s in terms of an x coordinate, and divides P(i,j) and P(i,j+1) so that t:1−t in terms of a y coordinate.
Here, (1−s)P(i,j)+sP(i+1,j) in the first term of the right side of the expression (1) indicates an A point of FIG. 8, that is, a pixel value of a point dividing P(i,j) and P(i+1,j) so that s:1−s. Further, (1−s)P(i,j+1)+sP(i+1,j+1) in the second term of the right side of the expression (1) indicates a B point of FIG. 8, that is, a pixel value of a point dividing P(i,j+1) and P(i+1, j+1) so that s:1−s. Further, Q(x,y) is a pixel value of a point dividing the A point and the B point so that t:1−t. Thus, the expression (1) is set.
In the linear interpolating method, in a case where a position of a pixel corresponding to Q(x,y) is identical to a position of the point P(i,j) of the original image, that is, in a case where s=t=0, the pixel value of P(i,j) is used as the pixel value of Q(x,y). While, in a case where a pixel corresponding to Q(x,y) is positioned at a central point of the internal area of the square surrounded by the four points P(i,j), P(i+1,j), P(i,j+1), P(i+1,j+1) of the original image, that is, in a case where s=t=0.5, an average value of the pixel values of these four points is the pixel value of Q(x,y).
As the pixel corresponding to Q(x,y) is positioned closer to any one of these four points P(i,j), P(i+1,j), P(i,j+1), P(i+1,j+1) of the original image in this manner, the pixel value of Q(x,y) tends to be influenced by the closest point. In this case, a value close to the pixel value is used as a pixel value of the scaled-up (down) image, so that the sharpness hardly varies.
While, as the pixel corresponding to Q(x,y) is positioned closer to the central point of the internal area of the square surrounded by the four points of the original image, the pixel value of Q(x,y) is a value influenced by pixel values of more pixels of the original image. In this case, values of some pixels are added to each other so as to calculate a pixel value of the scaled-up (down) image. Such a process corresponds to a process for smoothing the image, so that an image area calculated in this manner is an image whose sharpness is a little deteriorated.
That is, as Q(x,y) is positioned closer to the central point of the internal area of the square surrounded by the four points of the original image, noises contained in the original image are weakened due to the smoothing so as to be outputted. While, as the pixel corresponding to Q(x,y) is positioned closer to any one of the four points P(i,j), P(i+1,j), P(i,j+1), P(i+1,j+1) of the original image, the noises contained in the original image are outputted as they are.
Thus, as shown in FIG. 9, there occurs unevenness in the noise strength of the image data after scaling up (down). That is, in FIG. 9, a coordination position of each pixel is indicated as a horizontal axis, and the noise strength is indicated as a vertical axis.
Note that, FIG. 9 illustrates a case where the original image is scaled up by 1.25 times. That is, four sections divided by five pixels a1 to a5 (indicated by “∘” in FIG. 9) adjacent to each other in the original image are scaled up as five sections divided by six pixels b1 to b6 (indicated by “●” in FIG. 9) adjacent to each other in the scaled up image.
The pixels b1 and b6 of the scaled-up image are identical to the pixel a1 and a5 of the original image in terms of the coordination position, so that the noise strength of the scaled-up image is the same as in the original image. While, the pixels b2 to b5 of the scaled-up image are positioned closer to central points between the respective pixels, so that the noise strength is reduced.
Further, in FIG. 9, the sections having five pixels of the original image data and six pixels of the scaled-up image are extracted so as to be illustrated. That is, as to pixels of the entire image, a portion, in which the noise strength is high in the scaled-up image, and a portion, in which the noise strength is low in the scaled-up image, are brought about periodically.
In the image data of the scaled-up (down) image, unevenness in the noise strength occurs periodically, so that there occur a portion having many noises and a portion having less noises. Further, density seems to be different between the portion having many noises and the portion having less noises, so that there occurs density unevenness in the scaled-up (down) image.
Thus, in the scaled-up (down) image, there occurs such a problem that: the density unevenness brought about in the foregoing manner looks like striped patterns in a lattice manner (hereinbelow referred to as lattice noise). For example, in a case where a scaling down process by 97% is performed, the striped patterns are brought about at each pitch of about 5 mm.
Further, in a case where an analog image on a negative film is read as a digital image so as to perform the scaling-up (down) process with respect to the digital image for example, the following case is brought about.
That is, when an image on the negative film is in a state of underexposure (density of the entire image is slightly low), a process for strengthening contrast is performed so as to prevent the scaled-up (down) image from being blurred. The process for strengthening contrast causes the lattice noise brought about by the scaling-up (down) process to be conspicuous, so that the image quality is deteriorated.
While, in a case where the image on the film is in a state of overexposure (density of the entire image is slightly high), light that a CCD functioning as an imaging device reads from the film is little. Thus, an output of the CCD is comparatively low, and many electric noises are contained in the information, so that the lattice noise having been subjected to the scaling-up (down) process is conspicuous in the image information, thereby bringing about such a problem that the image quality is deteriorated.
In the case where the density of the entire input image is slightly high or low in this manner, the lattice noise brought about by the scaling-up (down) process is conspicuous, so that there occurs such a problem that the image quality is deteriorated.